278 ZOANTHARIA. 



close connection with the typical Cystiphylloidea, and, through these, 

 with the most abnormal of all the forms included in the Rugosa 

 (viz., Calceola and its allies). Upon the whole, therefore, in the present 

 condition of our knowledge, it seems best to retain the name Rugosa 

 for the three groups of the Zaphrentoid, Cyathophylloid, and Cysti- 

 phylloid corals, and to regard these as constituting a special division 

 of the Madreporaria. 



While the corallum of the Rugosa, as above limited, is in general 

 structure quite similar to that of the Madreporaria Aporosa, it pos- 

 sesses certain special peculiarities, which must be briefly noticed here. 

 The structure of the theca or wall of the corallum in the Rugosa is 

 generally in agreement with that of the Aporosa, the outer invest- 

 ment of the visceral chamber being formed by the coalescence of 

 lateral outgrowths from the peripheral margins of the septa, as seen, 

 for example, in Zaphrentis (fig. 127, c). In other cases (as in Helio- 

 phyllum and Crepidophylluni), the septa appear to end abruptly in 

 the wall, which is very thin, and seems to be developed indepen- 

 dently of the septa. In Streptelasma, again, a dense false wall is 

 formed by the junction of the much-thickened outer ends of the 

 septa (fig. 127, b). A true "epitheca" has been very commonly 

 described as present in the simple Rugose Corals, but it is very diffi- 

 cult to demonstrate the existence of this in microscopic sections, as 

 a structure distinct from the true wall, and it seems probable that 

 what has been generally called the "epitheca" is in many cases 

 really the "theca." In certain of the compound Rugose Corals, on 

 the other hand, there exists a general epitheca enclosing the entire 

 colony inferiorly. As regards their internal structure, the septa are 

 usually composed, as in the Aporosa generally, of a median plate 

 (" primordial septum ") bounded on both sides by a layer of dense 

 secondary sclerenchyma or " stereoplasma." In some cases, as in 

 Pholidophyllum, the septa can be shown to be formed by the coal- 

 escence of obliquely directed calcareous trabecular, the free ends of 

 which project at the inner margins of the septa. In Heliophyllum 

 and its allies, the septa are thin, and are either unthickened by 

 secondary stereoplasma, or have only a very thin layer of this sub- 

 stance on the sides of the primordial septum. In these cases the 

 oblique trabecular (" Septaldornen " of the Germans) above spoken of 

 are greatly developed, and give rise to the peculiar structures known 

 as " carinae," which will be more fully considered later on. 



The most important features in the structure of the corallum of 

 the Rugosa are, however, connected with the arrangement and mode 

 of development of the septa. The most obvious of the peculiarities 

 of a typical Rugose coral is the conspicuously bilateral disposition 

 of its parts (see fig. 139). The causes of this bilaterality will be best 

 understood by a consideration of the structure of a simple Rugose 



